1. Field of the Invention
The invention relates generally to the field of wireless communication; and in particular, to tunable duplexers and duplexing circuits as applied to RF Front-end configurations in wireless communication systems, and dynamic adjustment of bandwidth characteristics of RF circuits using such tunable duplexers and duplexing circuits.
2. Background Description
As the need for higher data rates increases, communication systems are being designed to cover wider frequency bandwidths as well as a larger number of frequency bands. The introduction of 4G protocols, such as Long Term Evolution (LTE), drive the increase in additional frequency bands being used for cellular communication systems. The complexity of the RF front-end topology of communication systems is increasing due to the need for backward compatibility with 2G and 3G protocols as 4G LTE capability is introduced. In addition, LTE-Advanced as a protocol is configured to accommodate carrier aggregation, where multiple channels can be transmitted or received-on simultaneously to increase instantaneous bandwidth. This aggregation of channels can cover up to five channels spread across multiple frequency bands. Carrier aggregation that utilizes multiple frequency bands points to a need for dynamic tuning of various components of the RF front-end to provide the flexibility needed to access various frequency pairings. All of these trends point toward a growing need for more flexibility in the RF front-end of mobile communication systems to address the combining of multiple frequency bands and modes.
Dynamic tuning of components that comprise the RF front-end of communication systems is picking up adoption in the commercial communications industry, and proper implementation of dynamic tuning methods can bring improvements to communication system performance as the number of frequency bands that can be accessed grows and the instantaneous bandwidths required increases.
Size constraints in mobile devices such as cell phones typically require a single antenna to cover multiple frequency bands. Additionally, frequency division duplex (FDD) protocols are commonly used at these multiple frequency bands to allow for both transmitter and receiver to utilize a common antenna. A typical frequency band used for communication will have a nominal 25 MHz allocated for transmitting, a 20 MHz guard band, and a 25 MHz frequency band used for receiving. The guard band is a frequency span set aside to provide frequency spectrum for the transmitted and receive signals to roll-off to minimize interference between transmitter and receiver. A duplexer is commonly used to allow for use of a single antenna by both the transmitter and receiver, while maintaining isolation between both transmitter and receiver ports. The duplexer is designed to provide the roll-off in the frequency response to produce the guard band situated between transmit and receive frequencies. Current duplexers are fixed in frequency and are not dynamically tunable. As more frequency bands are added to a communication system additional duplexers are used. The plurality of duplexers can be combined using a switch to allow for a single antenna to be used at multiple frequency bands by multiple duplexers. The requirement to design a mobile device such as a cell phone that covers multiple frequency bands and multiple modes of operation forces the system designers to develop a front-end transceiver circuit that combines several power amplifiers (PA) on the transmit side and several low noise amplifiers (LNA) on the receive chain. Typically, each PA and LNA is combined using a duplexer.
The duplexers used in current commercial communication systems have a fixed frequency response. The start and stop frequencies which establish the instantaneous bandwidth as well as the guard band are fixed. The conventional technique for implementing duplexers in a communication system is to determine the frequency bandwidth required from the duplexer along with the guard band bandwidth and the slope of the skirts (the roll-off in performance as a function of frequency) for a specific function and location within the circuit topology. A duplexer that meets the frequency response requirements is designed, manufactured, and implemented in the circuit. One main drawback is the inability to dynamically alter or tune the frequency response of the duplexer once it is implemented in a circuit. A tunable duplexer would provide the capability of dynamically adjusting the bandwidth of a transmit or receive circuit to track changes in bandwidth for LTE waveforms. LTE provides for a range of bandwidths for a data stream based upon the amount of data needed to transmit or receive and priority of the data stream in the cellular network. LTE bandwidths can vary from 1.5 MHz to 20 MHz. With carrier aggregation being implemented in LTE-Advanced (LTE-A), there is now a potential of up to five channels being aggregated to increase instantaneous bandwidth to 100 MHz. A tunable duplexer would provide the capability of matching the bandwidth of the communication system front-end to the instantaneous bandwidth of the LTE waveform.
Active antennas are becoming more prevalent in communication systems. These active antennas provide the capability of dynamically adjusting the impedance match of the antenna and/or the frequency response of the antenna. Examples of such active antennas are described in commonly owned patents: U.S. Pat. No. 7,830,320; U.S. Pat. No. 7,911,402; U.S. Pat. No. 8,077,116; U.S. Pat. No. 8,362,962; U.S. Pat. No. 8,648,755; and U.S. Pat. No. 8,717,241; the contents of which are hereby incorporated by reference. With the advent of 4G LTE protocols and the frequency band requirements that come with it, the active antenna that can be dynamically tuned provides an ability to cover a wider frequency range from that of a small form factor antenna by dynamically tuning the antenna to re-tune for an intended frequency band. With the capability of changing the frequency response of an active antenna there is a potential of providing better frequency management from the entire RF front-end of a communication device, such as a cell phone, by implementing a tunable duplexer and tuning both the duplexer and active antenna con-currently as a system.
The embodiments disclosed herein address the aforementioned problems and provide a unique method of tuning a duplexer to provide dynamic adjustment of the frequency of operation and bandwidth.